Hitherto, as a rotational number-detecting apparatus used for anti-skid for preventing skidding of an automobile or used for a traction control for effectively transmitting a drive force to a road face, there has been frequently used an apparatus constituted by a magnetic encoder for generating a pulse by magnetism and a detection sensor for detecting the magnetic pulse of the magnetic encoder. As the rotational number-detecting apparatus, a seal fitted with the rotational number-detecting apparatus is commonly used. The apparatus is constituted by arranging the magnetic encoder along with a hermetically sealing apparatus for hermetically sealing a bearing to thereby integrate a hermetically sealing means and a rotational number-detecting means (see, for example, Patent Document 1).
One example of the seal fitted with the rotational number-detecting apparatus is shown in FIG. 13, which is constituted by a seal member 202 attached to an outer ring 201a, a slinger 203 fitted to an inner ring 201b, a magnetic encoder 204 attached to an outer side face of the slinger 303 for generating a magnetic pulse, and a sensor 205 arranged proximately to the magnetic encoder 204 for detecting the magnetic pulse. According to a bearing unit attached with the sealed rotational number-detecting apparatus, a foreign matter of dust, water or the like is prevented from invading inside of the bearing by the seal member 202 and the slinger 203, and a lubricant filled at inside of the bearing is prevented from leaking to outside of the bearing. Furthermore, the magnetic encoder 204 generates the magnetic pulses in a number corresponding to the number of the poles during a time period in which the inner ring 201b is rotated once, and detects a rotational number of the inner ring 201b by detecting the magnetic pulses by the sensor 205.
Moreover, in the magnetic encoder 204, a magnet portion formed of an elastic magnetic material obtained by mixing a magnetic powder with an elastic material such as rubber or a resin is bonded to a flange portion 203a of the slinger 203, to which an adhering agent is applied in a die, by press molding. As the elastic magnetic material, ferrite-containing nitrile rubber is generally used, and the magnetic powder is in a mechanically oriented state, which is achieved by kneading the powder with a roll.
Patent Document 1: Japanese Patent Unexamined Publication JP-A-2001-255337